In recent years, there has been increasing demand for high performance insulating windows. These windows typically include two or more sheets of rigid, transparent glazing material and may also include one or more sheets of nonrigid transparent material all held in parallel alignment to one another by an edge-seal system. This edge-seal may include spacer frame elements to position the glazing sheets relative to one another and a sealant to prevent moisture from entering and condensing in the voids between the glazing sheets.
A basic double glazing unit of the art is shown in FIG. 5 to include glazing sheets 12 and 14 (typically glass), and spacer 24 with a layer of adhesive 28 sealing the perimeter of the unit to keep out moisture which otherwise would condense on the internal surfaces of the glazing sheets.
A more advanced multipane glazing unit of the art is shown in FIG. 6. In this unit, glass sheets 12 and 14 and plastic film 16 make up three parallel glazing surfaces and define air or gas spaces 18 and 22. Sheets 12 and 14 and film 16 are spaced from one another by spacers 24 and 26 and the edge of the unit is sealed with adhesive 28. Typically, in both cases this sealant 28 is an elastomeric adhesive material which adheres to the sheets of glazing and helps to join then to the spacers. As the performance of these windows has improved, they have been employed in applications of ever-increasing harshness.
In these harsher environments, these windows often fail prematurely. Impact of sunlight on the sealant/adhesive (such as the impact of Rays R.sub.o and/or R.sub.1 28 in the prior art drawings) can have the effect of cross-linking and hardening the sealant. This can lead to embrittlement and a breakdown in the bond of the sealant to the glass panes and other components. One approach to solving this problem has been to use silicone materials as adhesive sealants. Silicones are quite resistant to light-induced cross-linking and hardening but have the serious failing that they are very readily permeated by water vapor. This leads to moisture condensing and collecting within the window structure. The solution to this moisture problem is to employ a two layer-two material seal system. The application of the seal systems is time consuming, labor intensive, and high priced.
Alternatively, especially when using organic sealants such as polyurethanes and polysulfides, this problem has been avoided here-to-fore at least in part by encasing the edge of the units in a mullion cap. Such a cap 32 is held in place by foam adhesives 34 and 36 in the prior art FIG. 6. These caps have been used for their architectural and fabrication properties but have also shielded the sealant/adhesive from the direct rays of the sun such as ray R.sub.o shown in the two prior art figures which is seen entering the sealant in FIG. 5.
The use of mullion caps does to some extent protect the adhesive/sealant, but certain practical problems prevent these from being completely effective in many applications. The caps are easily dislodged and forced out of alignment, they often do not fit flush to the outside of the glass and they can lend themselves to poor alignment due to installation error or poor engineering design. See, for example, the gap shown in the prior art figure.
The use of mullion caps has helped but has had problems. The caps are expensive, they are easily dislodged and forced out of alignment and also, they often do not fit flush to the outside of the glass. See, for example, the gap shown in the prior art figure. Typically, this gap was not considered to be a problem. Recently, however, increasing failure rates have been noted for seals in windows as shown in this figure. In addition, these mullion caps primarily serve to block direct incident exposure (rays R.sub.o and R.sub.1) and do not take into account that there is substantial amounts of light reaching the sealant through internal reflection within the outer glazing sheet itself. Such light as shown as rays R.sub.3 and R.sub.5 in FIG. B.